This invention relates generally to aircraft navigation, and more specifically, to installation of inertial navigation and attitude measuring devices within vehicles such as aircraft, or any other system utilizing an attitude measuring device.
When an attitude measuring or inertial measurement device, for example, an Attitude Heading Reference System (AHRS), Inertial Reference System (IRS), or Inertial Measurement Unit (IMU) is installed in a vehicle there is the need to accurately determine and correct for a physical misalignment between the actual installation alignment and the desired installation alignment, which is typically along the axes of a vehicle. Such measuring and measurement devices are collectively referred to herein as inertial reference units, or IRUs. Typically, a difference between actual and desired installation alignment is further complicated since the IRUs typically are mounted on a fixture, or jig, which is mounted to the vehicle. The jig installation alignment therefore introduces an additional source of installation error. In addition, different vehicles, for example, different aircraft, are designed such that IRUs are mounted in each aircraft in a different orientation compared to the mounting orientation in other aircraft. Therefore, different alignment methods are needed for each mounting orientation in order to provide accurate attitude (roll, pitch, and heading) information.
The desired IRU installation alignment is typically with respect to the vehicle principle axes. Typically, the principle axes for an aircraft (illustrated in FIG. 1) are defined to be out of the nose, right wing, and down. The misalignment between the installed device and the vehicle principle axes can be determined in several ways, depending on available equipment, tools, and reference information.
One method for aligning an IRU with the vehicle axes is often referred to as a manual alignment method. The manual alignment method requires that the vehicle be leveled along its principle axes, for example, the roll and pitch axes, and further requires an external heading reference point. One type of external heading reference point is referred to as a compass rose, which provides known heading references. The vehicle is aligned, while level, with the compass rose at one of the known headings. Tools are used, for example, a plumb-bob or a laser device, to ensure that the vehicle is accurately aligned with the compass rose. The IRU readings are then compensated, typically by coefficients in a memory, so that the measured vehicle attitude (roll, pitch, and yaw) agrees with the external reference heading and the vehicle level condition.
Another method of aligning the aircraft and the IRUs is generally referred to as a transfer alignment. In the transfer alignment method, the vehicle is considered to have a xe2x80x9ctruth spotxe2x80x9d, providing a truth reference with an accompanying reference jig. The reference jig has a reference IRU mounted upon it, and as certain vehicles have more than one installed IRU, the reference IRU is moved to and from the multiple mounting locations. This method includes procedures for compensation in alignment measurement differences received as the reference IRU is moved from a first mounting location (i.e. the xe2x80x9ctruth spotxe2x80x9d) to other mounting locations.
Another alignment method currently used is often referred to as nose plunge. The known nose plunge method has a prerequisite of physically leveling the vehicle in the principle axes and then physically or electronically adjusting the installed device until it also indicates level. Once the device and the aircraft both indicate level, the aircraft is then rotated about one of the principle axes that is now level, for example, pitch, so as to determine the misalignment about a vertical axis, which is sometimes referred to as a yaw misalignment.
The existing nose plunge method only produces a yaw misalignment correction since this method requires that the IRU being used to collect the roll and pitch measurements first be corrected, that is provide compensated readings, in the level (roll and pitch) axes. Compensation is one of an electronic compensation, for example, in a memory, or a physical installation of shims to compensate for misalignment with the principle axes. The compensation encompasses an additional installation method to be performed before the yaw misalignment can be determined via nose plunge.
The three alignment methods described above include drawbacks. At least two drawbacks are the reliance on the aircraft being nearly level in two axes, and the need for external equipment and facilities. Such equipment and facility examples include, the compass rose (or an equivalent) and the reference jig and IRU. However, such equipment and facilities are not common at all aircraft construction and maintenance facilities where an IRU may be installed or replaced.
In one aspect, a method for installation alignment of an inertial reference unit (IRU) with vehicle axes, the vehicle axes including roll, pitch, and yaw axes, is provided. The provided method comprises recording vehicle angular position data, including roll and pitch, using an angular position measurement device, the vehicle being in a starting position. The method also comprises recording IRU data, including roll and pitch, assuming a heading reference value, the assumed value to be included with both the vehicle angular position data and the recorded IRU data, and receiving measured nose plunge data. The method continues as initial roll and pitch misalignment corrections are computed, initial roll and pitch misalignment corrections are applied to measured nose plunge data, and a nose plunge yaw misalignment is determined using the measured nose plunge data to adjust the assumed heading reference.
In another aspect, an apparatus configured for installation alignment of an inertial reference unit (IRU) with vehicle roll, pitch, and yaw axes is provided. The apparatus comprises a vehicle angular position measurement device and a computer which is configured to be coupled to the angular position measurement device and the IRU. The computer is configured to record a condition of the vehicle in the roll and pitch axes, the vehicle being in a starting position, through roll and pitch readings received from said angular position measurement device. The computer is also configured to record IRU data including roll and pitch received from the IRU at the starting position, assume a single heading value for both said angular position measurement device data and the IRU data, record IRU data including roll, pitch, and heading received from the IRU at multiple nose plunge positions, and compute initial roll and pitch misalignment corrections. The computer further applies initial roll and pitch misalignment corrections to data received from the IRU at the multiple nose plunge positions, and determines a nose plunge yaw misalignment using the IRU data from the multiple nose plunge positions to adjust the heading value.